The present invention relates to an optical character recognition apparatus, and more particularly to a binary signal producing apparatus which produces an optimum binary signal from a multi-value image signal obtained from a scanner.
A conventional method of two-dimensionally processing a multi-value image signal, in a real-time manner, employs a mask constructed by a n.times.n matrix. In this method, the value of a center element in the mask is compared with the average of the values of elements which are contiguous to the center element. Thus, the center element is determined as black when the value of the center element is smaller (darker) than the average of the values of contiguous elements and vice versa. FIG. 1 shows an example of such mask constructed by a 5.times.5 matrix.
The conventional binarization method can provide a good result when a suitable size of a mask is selected, but only so long as the size of characters to be recognized and the width of their strokes are uniform. However, the conventional method encounters some problems when there are large variations in the character size and the stroke width, for instance, when a postal code and an address description on a mail article are recognized. One of the problems is that, when a stroke width of the character is larger than the width of the mask, the value of the center element could become substantially equal to the average of the values of the contiguous elements. Thus, the center element is frequently determined as white. FIG. 2 illustrates this problem. In this figure, the width of the mask M is smaller than the stroke width of the character. Therefore the central portion of the stroke is errorneously determined as white. This phenomenon is known as "center line omission" and could seriously affect the character recognition. Reversally, FIG. 3 show a character pattern corresponding to an optimum binary signal.
Further, even when a mask has a sufficiently large size with respect to the size of the character, there is a problem because a portion of a character is erroneously interpreted as white under the influence of surrounding areas.